DESI is used in mass spectrometry to obtain ions directly from sample surfaces. For samples at or near atmospheric pressure, a charged aqueous solvent mixture or other fluid is electrosprayed with pneumatic assistance and directed at a sample surface. The spray interacts with analytes on the surface and produces ions (sometimes the ions are already present in the sample), some of which are adsorbed by the solvent droplets, sampled into the mass spectrometer, and analyzed for their mass to charge ratio. With the typical DESI source the signal intensity depends strongly on geometric factors including the angle and distance of the sprayer to the surface and those between the surface and the mass spectrometer inlet. The Optimum geometry is also dependent on the analyte and the sample surface. This requires re-optimizing of various parameters between different samples and causes uncertainties when comparing relative intensities of analytes obtained from different samples. As is the case for electrospray ionization (ESI), only a small fraction of the divergent analyte containing spray is sampled into the mass spectrometer largely because of inefficient collection at the atmospheric pressure interface. In DESI, droplet scattering occurs at the surface and this further reduces the droplet sampling efficiency. The sample is typically open to the atmosphere of the laboratory during DESI and other ambient ionization methods, and this allows for easy manipulation of the surface during analysis. Concurrently, this open geometry potentially introduces solvent vapors into the laboratory atmosphere as well as sample components such as chemicals and biological materials when these are present on the surface. The high nebulizing gas pressure used in DESI means that in the case of biological samples, aerosols may be produced during the ionization process.
Moving mass spectrometers out of the lab into the field requires two key advances: 1) removal of arduous sample preparation steps, and 2) producing mass spectrometers that are small, portable and cheap. DESI is a giant leap towards removing sample preparation from mass spectrometric analysis. Reducing the size of mass spectrometers is hampered by the requirement for mass spectrometry to be performed in vacuum. Coupling DESI to a mass spectrometer requires an atmospheric pressure—vacuum interface with a large pumping capacity to deal with the fact that the vacuum system needs to combat the continuous influx of air. Thus, DESI and mini-mass spectrometers are not natural partners.
Most atmospheric pressure desorption ionization experiments depend on optimization of instrumental geometry as well as requiring chemical preparation steps. For example, atmospheric pressure matrix assisted laser desorption requires meticulous care in matrix deposition. Atmospheric pressure matrix free laser desorption ionization has not yet been reported, although electrospray assisted laser desorption ionization will potentially make this possible. The liquid micro-junction probe/ESI emitter depends heavily on the maintenance of an optimum liquid junction thickness requiring a skilled operator or computer control. In DESI too, although sample preparation is generally not used, signal intensity depends on such chemical factors such as the spray solvent and surface polarities and the analyte identity. Signal intensity also depends on physical factors such as the sizes and velocities of incident droplets, sample surface roughness and porosity and, most significantly, on various geometric factors such as the spray angle, the collection angle and the distances of the sprayer and collecting capillaries from the sample surface. DESI has been implemented using various mass spectrometers including triple quadrupoles and linear ion traps, quadrupole-time-of-flight (QTOF) instruments, ion mobility/TOF and ion mobility/QTOF hybrids, and Fourier transform ion cyclotron resonance instruments, among others. While optimization depends on the particular instrument and DESI source used, certain trends are usually observed.